A perilymph fistula is an abnormal connection, i.e., a small hole in the round or oval window, between the perilymph fluid of the inner ear and the middle ear air space. The first described cases of perilymph fistulas were a consequence of otic capsule bone erosion associated with middle ear and mastoid infections (Schuknecht, 1974). Fee (1968) was the first to report oval and round window fistulas caused by head trauma. Patients with both types of perilymph fistulas report episodes of dizziness, vertigo, dysequilibrium, tinitus, and hearing loss. Other abnormal couplings between the air-filled middle ear and the fluid-filled inner ear involve softening of the otic capsule, evulsion of the stapes footplate, deformation of the ossicular chain, and fractures of the ossicles or temporal bone. In some cases these problems are congenital rather than adventitious.
A reliable and non-invasive clinical test for perilymph fistulas and other abnormal communication between the middle and inner ear is not possible with the devices and methods currently available to clinicians. Because the history and symptoms reported by the patient with a middle ear perilymph fistula often resemble those reported by patients with other forms of inner ear vestibular disorders, symptoms and history do not provide a reliable means to determine whether or not the patient has a perilymph fistula. According to a number of clinicians specializing in vestibular disorders, a definitive diagnosis of a post-traumatic middle ear perilymph fistula can be made only by direct inspection of the middle ear through surgical intervention, i.e., tympanotomy (Goodhill, 1980; Healy, 1974 & 1976; Simmons, 1982; Lehrer, 1984; Kohut, 1979; Nomura, 1984; Singleton, 1978). Hence, a reliable non-invasive perilymph fistula test would eliminate the need for exploratory surgery in many cases.
Several groups of individuals have attempted to develop non-invasive and yet reliable tests for the presence of middle ear perilymph fistulas. Daspit, et al., (1980) used an impedance bridge to introduce controlled changes in external ear canal pressure while recording the patient's eye movement responses using electronystagmography. These clinicians used air pressure stimuli ranging from -600 +300 millimeters of water. Supance (1983) and Healey, et al. (1979) used similar techniques. In all of these instances, however, the reliability of the test was found to be relatively low, ranging from a high of 75% to a low of 37%. Noting the high incidence of postural instability among fistula patients, Lehrer, et al. (1984) used the so called Quix test in which body sway responses to external ear canal pressure stimuli are measured with the subject standing freely. In the Quix test, however, the subject stands on a fixed surface and within a normally fixed visual surround. Hence, this method does not test the subject's postural response to external ear canal pressure at a time when the posture control system is maximally sensitive to vestibular inputs.
A number of signal processing methods exist for determining whether or not a stimulus produces statistically significant changes in a measured variable. One such technique is termed "pulse triggered averaging". The stimulus in this technique consists of a train of discrete mono or biphasic pulses. The measured variable is divided into segments that are time locked to the onset of the stimulus pulses. The segments are then averaged. A significant change in the properties of the measured variable correlated with the onset of pulses indicates that the measured variable is influenced by the stimulus. A second technique uses continuously varying sinusoidal or step-like stimuli and "linear systems analysis" to determine whether or not temporal properties of the measured variable are significantly correlated with the stimulus (for example; Brown, et al., 1982).